Implement TryFrom<char> for usize.

[bjoernager]

Feature gate: usize_try_from_char.

This PR implements TryFrom<char> for usize.

usize is currently the only, unsigned, integral, type to not implement TryFrom<char>. Technically, this conversion is trivial and can already be expressed with some indirection.

I think it useful to be able to describe this set of types with this interface.

[rustbot]

Failed to set assignee to ibraheemdev: cannot assign: error sending request

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[rustbot]

r? @ibraheemdev

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[bjoernager]

@rustbot label +T-libs-api -T-libs needs-fcp

r? libs-api

[dtolnay]

Thanks!

View changes since this review

[dtolnay]

@rfcbot fcp merge

[rust-rfcbot]

Team member @dtolnay has proposed to merge this. The next step is review by the rest of the tagged team members:

• @Amanieu
• @BurntSushi
• @dtolnay
• @joshtriplett
• @the8472

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